The present invention relates to an improved method for the recovery of oil from subterranean hydrocarbon-bearing formations containing low API gravity viscous oils or bitumen. More particularly, the invention relates to the production of bitumen and hydrocarbons from reservoirs of low mobility, such as tar sand formations.
The recovery of viscous oils from formations and bitumen from tar sands has generally been difficult. Although some improvement has been realized in stimulating recovery of heavy oils, i.e., oils having an API gravity in the range of 10.degree. to 25.degree. API, little, if any, success has been realized in recovering bitumen from tar sands. Bitumen can be regarded as highly viscous oils having a gravity in the range of about 5.degree. to 10.degree. API and contained in an essentially unconsolidated sand referred to as tar sands.
Vast quantities of tar sands are known to exist in the Athabasca region of Alberta, Canada. While these deposits are estimated to contain several hundred billion barrels of oil or bitumen, recovery therefrom using conventional in-situ techniques has not been too successful. The reasons for the lack of success relate principally to the fact that the bitumen is extremely viscous at the temperature of the formation, with consequent low mobility. The viscosity of the tar sands from the Athabasca deposits, for example, is in the range of several million centipoise at the average formation temperature of about 40.degree. F, so that the bitumen is essentially immobile at formation temperature. In addition, these tar sand formations have very low permeability, despite the fact they are unconsolidated.
Since it is known that the viscosity of oil decreases markedly with an increase in temperature, thereby improving its mobility, thermal recovery techniques have been investigated for recovery of bitumen from tar sands. These thermal recovery methods generally include steam injection, hot water injection and in-situ combustion.
Typically, such thermal techniques employ an injection well and a production well traversing the oil-bearing or tar sand formation. In a steam operation employing two wells, steam is introduced into the formation through the injection well. Upon entering the formation, the heat transferred by the hot fluid functions to lower the viscosity of oil, thereby improving its mobility, while the flow of the hot fluid functions to drive the oil toward the production well from which it is produced.
In the conventional forward in-situ combustion operation, an oxygen-containing gas, such as air, is introduced into the formation via a well, and combustion of the in-place crude adjacent the wellbore is initiated by one of many known means, such as the use of a downhole gas-fired heater or downhole electric heater or chemical means. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain a combustion front which is formed and to drive the front through the formation toward the production well.
As the combustion front advances through the formation, a swept area consisting, ideally, of a clean sand matrix is created behind the front. Ahead of the advancing front various contiguous zones are built up that also are displaced ahead of the combustion front. These zones may be envisioned as a distillation and cracking zone, a condensation and vaporization zone, an oil bank and a virgin or unaltered zone.
The temperature of the combustion front is generally in the range of 750.degree.-1100.degree. F. The heat generated in this zone is transferred to the distillation and cracking zone ahead of the combustion front where the crude undergoes distillation and cracking. In this zone a sharp thermal gradient exists wherein the temperature drops from the temperature of the combustion front to about 300.degree.-450.degree. F. As the front progresses and the temperature in the formation rises, the heavier molecular weight hydrocarbons of the oil become carbonized. These coke-like materials are deposited on the matrix and are the potential fuel to sustain the progressive in-situ combustion.
Ahead of the distillation and cracking zone is a condensation and vaporization zone. This zone is a thermal plateau and its temperature is in the range of from about 200.degree. F to about 450.degree. F, depending upon the pressure and the distillation characteristics of the fluids therein. These fluids consist of water and steam and hydrocarbon components of the crude.
Ahead of the condensation and vaporization zone is an oil bank which forms as the in-situ combustion progresses and the formation crude is displaced toward the production well. This zone of high oil saturation contains not only reservoir fluids, but also condensate, cracked hydrocarbons and gaseous products of combustion which eventually reach the production well from which they are produced.
Various improvements relating to in-situ combustion are described in the prior art that relate to the injection of water, either simultaneously or intermittently with the oxygen-containing gas, to scavenge the residual heat in the formation behind the combustion front, thereby increasing recovery of oil. Prior art also discloses regulating the amount of water injected so as to improve conformance or sweep efficiency.
Experience has generally shown that these conventional thermal techniques have not been altogether successful when applied to the recovery of heavy oils or bitumen. Where the hydrocarbons sought to be produced have a low API gravity, the build-up of the oil bank ahead of the thermal front occurs to a great extent. Since the heat transfer is low ahead of the front, these heavy hydrocarbons become cool and hence immobile, thereby causing plugging of the formation with the result that the injection of either air in the case of in-situ combustion, or steam in the case of steam, is no longer possible.
The problems recited above become compounded when these techniques are applied to the tar sands not only because of the very low API gravity and very high viscosity of the bitumen, but also because of the very low permeability of the tar sand formations.
Accordingly, it is an object of the present invention to provide an improved thermal recovery method whereby both highly viscous, low gravity crudes and bitumen can be recovered more efficiently. The instant invention accomplishes this recovery of heavy oils and bitumen by utilizing thermal methods wherein a mixture of an oxygen-containing gas and steam is injected having an optimum ratio of gas-to-steam that is controlled in relation to the cumulative amount of steam injected.